1. Field of the Invention
The present invention relates to an X-ray mask structure used for printing a fine pattern by X-ray exposure on a wafer for a large-scale integrated circuit (LSI), microoptics, and micromachines. The present invention further relates to a process for producing the X-ray mask structure, an X-ray exposure apparatus and X-ray exposure method employing the X-ray mask structure, and a semiconductor device produced by the X-ray exposure method.
2. Related Background Art
Large scale integrated circuits typified by DRAM are now in a mass-production period of 4 M (mega) DRAM, and are advancing rapidly to 16 M DRAM, and further to 256 M DRAM. With the progress, the minimum line width required for the semiconductor device is becoming smaller, on the order of a half micron or a quarter micron.
In the production of such a semiconductor device, a mask pattern is printed on a semiconductor substrate by use of light having a wavelength of near ultraviolet to far infrared. However, the printing line width practicable with such a wavelength of light is coming close to the limit. Further, shortening of the exposure light wavelength, and increase of the numerical aperture unavoidably results in a decrease of focal depth. Therefore, an X-ray exposure apparatus is being developed which utilizes light (2 to 20 .ANG.) of an X-ray region as the exposure light wavelength. The lithography technique employing X-rays is promising for solving the above problems simultaneously.
The X-ray mask structure generally has an X-ray transmissive film formed on a suitable supporting frame, and thereon a fine pattern of an X-ray absorbing material or an alignment pattern for use for alignment.
The illustrations for producing the aforementioned X-ray mask structure are shown below.
A first illustration, which is characterized by a flattening treatment of the X-ray transmissive film, is described by reference to FIGS. 21A to 21G.
Firstly, X-ray transmissive films 12a and 12b are formed on a top face and a back face of an Si substrate 11 as shown in FIG. 21A. An SiC or SiN film formed by a chemical vapor deposition (CVD) or the like method, for instance, is used as the X-ray transmissive film. Such a film is crystalline and has a fine surface roughness as shown in the drawing schematically. The surface roughness is disadvantageous to meet the requirements for high performance of semiconductor devices.
Then, the central portion of the X-ray transmissive film 12a on the back face of the Si substrate 11 is etched to leave the peripheral portion of the X-ray transmissive film 12a to form a ring-shaped film 12c as shown in FIG. 21B.
The central portion of the Si substrate 11 is etched by using the ring-shaped film 12a as the mask to leave the peripheral portion 11a of the Si substrate 11 as shown in FIG. 21C.
Subsequently, as shown in FIG. 21D and FIG. 21E, the surface of the X-ray transmissive film 12b on the top face of the Si substrate 11 is subjected to a flattening treatment. In the flattening treatment, a flattening-treatment film 14 is formed on the X-ray transmissive film 12b as shown in FIG. 21D: for instance, an SiO.sub.2 film as the flattening treatment film on an X-ray transmissive film composed of SiC. Then, the entire surface of the flattening treatment film 14 is etched uniformly such that the flattening treatment film 14 is completely removed from the upper surface of the X-ray transmissive film 12b as shown in FIG. 21E. When the flattening treatment film 14 has been completely etched off, the flattening treatment is completed to form a flattened X-ray transmissive film 12d.
Further, an X-ray absorbent film 15 is formed on the flattened X-ray transmissive film 12d as shown in FIG. 21F, and a pattern 15a of the X-ray absorbent film 15 is formed in a conventional manner. Thus, the entire process of the production of the X-ray mask structure has been finished to obtain an X-ray mask structure having a desired pattern.
A second and a third illustration are characterized by the formation of a pattern of an X-ray absorbing material. The second illustration concerns the formation of an X-ray absorption pattern by metal plating, and the third illustration concerns that by etching.
The second illustration is described by reference to FIGS. 22A to 22H.
Firstly, an X-ray transmissive film 42 is formed from silicon nitride or the like by chemical vapor deposition (CVD) on a silicon wafer 41 which has been polished on both faces and will serve later as the supporting frame, as shown in FIG. 22A.
Then, the silicon wafer 41 is etched from the backside to form a non-supported film portion of the X-ray transmissive film 42 in the region on which a pattern is to be formed as shown in FIG. 22B.
On the X-ray transmissive film 42, a Cr film 43, and an Au film 44 for a plating electrode are vapor-deposited in a thickness respectively of 5 nm and 50 nm continuously by an electron-beam (EB) deposition apparatus as shown in FIG. 22C.
On the metal film, a resist 45 is applied and a pattern of the resist is formed by an optical exposure means such as a stepper, or an electron-beam drawing apparatus as shown in FIG. 22D.
Into the gaps in the resist pattern, heavy metal 46 such as Au is deposited as shown in FIG. 22E, and thereafter the resist is stripped off.
Then, the uncovered electrode portion on the non-patterned area is treated for transparency by Au sputtering and chrome oxidation by O.sub.2 -RIE as shown in FIG. 22F to obtain an X-ray mask structure.
Finally, this substrate is bonded to a supporting frame 47 made of Pyrex glass, silicon carbide, titanium alloy, or the like with an adhesive 48, e.g., of epoxy type, as shown in FIG. 22G.
If the X-ray mask structure is to be held with a magnetic chuck on an X-ray exposure apparatus, a magnetic body 49 is further bonded to the mask frame 47 as shown in FIG. 22H.
The third illustration is described below by reference to FIGS. 23A to 23H.
Firstly, an X-ray transmissive film 52 is formed from silicon nitride or the like by chemical vapor deposition (CVD) on a silicon wafer 51, which has been polished on both sides and will serve later as the supporting frame, as shown in FIG. 23A.
Then, the silicon wafer 51 is etched from the backside to form a non-supported film portion of the X-ray transmissive film 52 in the region on which a pattern is to be formed, as shown in FIG. 23B. On the X-ray transmissive film, an etching stopper layer 53 and an X-ray absorption film 54 are respectively formed by sputtering as shown in FIG. 23C.
Further thereon, a resist 55 is applied such as an electron-beam resist like PMMA and a photoresist. A pattern of the resist is formed by an optical exposure means such as a stepper, or an electron-beam drawing apparatus as shown in FIG. 23D.
By use of this resist as a mask, the X-ray absorption film 25 is dry-etched to form a pattern as shown in FIG. 23E.
Then, the uncovered etching stopper layer 53 on the non-patterned area is removed to obtain an X-ray mask structure having a desired pattern on the X-ray transmissive film as shown in FIG. 23F.
Finally, this substrate is bonded to a supporting frame 56 made of Pyrex glass, silicon carbide, titanium alloy, or the like with an adhesive 57 such as of epoxy type as shown in FIG. 23G.
If the X-ray mask structure is to be held with a magnetic chuck on an X-ray exposure apparatus, a magnetic body 58 is further bonded to the mask frame 56 as shown in FIG. 23H.
In an X-ray lithography technique employing the above X-ray mask structure, precise positional registration is important between the X-ray mask structure and a substrate for light exposure.
FIG. 24 illustrates schematically an ordinary X-ray exposure apparatus employing an X-ray mask structure, having a synchrotron source 60 as the X-ray source, an X-ray reflection mirror 61 for spreading the X-ray beam in a vertical direction, an X-ray mask structure 62, and a silicon wafer 63 as a substrate for exposure.
FIG. 25 schematically shows positional registration of an X-ray mask to a wafer. In FIG. 25, reference numeral 22 denotes an X-ray mask structure; 23 denotes a substrate, a silicon wafer, to be exposed; 24 to 31 denote alignment patterns on the X-ray mask; 16 to 20, and 32 and 33 denote parts constituting an optical alignment system. In one method for the alignment, a light diffraction phenomenon in an alignment pattern is utilized.
Specifically, a special optical alignment system provided on a light exposure apparatus detects a positional deviation of the X-ray mask structure 22 from the wafer 23, the two of which are placed at an extremely small spacing of several .mu.m to several tens of .mu.m by utilizing the alignment patterns 24 to 31 written on the X-ray mask pattern structure 22 and an alignment pattern (not shown in the drawing) formed preliminarily on the silicon wafer 23. According to the detected deviation, the stage of the light exposure apparatus is driven to obtain precise alignment between the X-ray mask structure and the wafer.
After completion of the alignment, the pattern on the X-ray mask structure is transcribed onto the wafer by X-ray exposure. A semiconductor device or the like is produced with a plurality of X-ray masks and a usual semiconductor production process in such a manner.
The X-ray mask structure prepared by the above-described process involves disadvantages discussed below.
In the above first illustration of a production process of an X-ray mask structure, the etching conditions need to be set to make the etching rate of the treatment film 14 equal to that of the X-ray transmissive film 12b. An error in the condition setting may cause incomplete flattening of the X-ray transmissive film 12b. In particular, SiC as the X-ray transmissive film 12b cannot readily be flattened by the aforementioned flattening treatment because of the many grain boundaries appearing on the surface owing to the polycrystalline nature of SiC.
Therefore, when lithography is conducted with an insufficiently flattened X-ray transmissive mask structure, or when a substrate surface is worked by X-ray exposure through the mask structure, alignment light or an X-ray introduced to the X-ray mask structure is scattered at the surface of the insufficiently flattened surface. The scattered light partly enters the detector of the optical alignment system to lower the precision of the alignment to impair the precision of the working by X-ray lithography.
In the X-ray mask structure of the second illustration, the X-ray absorption pattern or the alignment pattern is formed, for instance, by crystal growth of an Au layer by metal-plating as shown in FIGS. 22A to 22H. Therefore, the crystal growth face, namely the top face of the pattern 46 has the fine roughness of a grain boundary of the Au crystal.
On the other hand, the lateral wall of the pattern, which is an accurate reproduction of the resist pattern 46 employed at the metal plating, also has fine irregularity caused by fine irregularity of the lateral wall of the resist owing to the instability of the stationary wave or of the EB exposure beam, dissolution rate variation in the resist development, and other causes. Furthermore, the back face of the X-ray absorption pattern also has fine irregularity caused by the surface roughness of the X-ray transmissive film or the underlying substrate.
In the case when an X-ray absorption pattern or the alignment pattern is formed by etching as mentioned in the third illustration shown in FIGS. 23A to 23H, the X-ray absorbent film 53, which is formed by sputtering of tantalum, tungsten or the like, has fine roughness on the surface, and the lateral wall of the X-ray absorption pattern formed by etching also has fine irregularity owing to pattern edge roughness after the resist process, etching conditions, and other factors. Further, the back face of the X-ray absorption pattern also has fine irregularity caused by the surface irregularity of the X-ray transmissive film or the underlying substrate.
When optical alignment is practiced with such an X-ray mask structure having fine roughness of the pattern placed close to the wafer by means of an X-ray exposure apparatus, the alignment light is scattered at the surface of an X-ray absorption pattern or an alignment pattern. This scattered light partly enters the detector of the optical alignment system to decrease the precision of the working by X-ray lithography, which is a serious problem.
This problem is described below in more detail by reference to FIGS. 27A to 27C. In the drawings, the numerals 501, 511, and 521 denote respectively a supporting frame of the X-ray transmissive film. FIGS. 27A to 27C show examples of the scattering modes of alignment light from an X-ray absorption pattern and an alignment pattern.
FIG. 27A shows scattering of incident alignment light at the back face of the pattern 503 (namely the face in contact with the X-ray transmissive film 502). The scattered light 506, after multiple reflection at the back face of the pattern or in the X-ray transmissive film 502, reaches the alignment light detector.
FIG. 27B shows scattering of incident alignment light 515 at the top face of a pattern 513 (namely the surface of the pattern confronting an exposure substrate 514). The scattered light 516, after multiple reflection between the surface of the wafer 514 and the X-ray absorption pattern 513 or the X-ray transmissive film 512, reaches the alignment light detector.
FIG. 27C shows scattering of incident alignment light 525 at the lateral wall of the pattern 523. This scattered light 526, after multiple reflection between the wafer 524 and the X-ray absorbent film or the X-ray transmissive film, reaches the alignment detector.
The scattered light as shown by the numerals 506, 516, or 526 in FIGS. 27A to 27C which enters the optical alignment system as shown by numerals 33a and 33b in FIG. 25 causes much noise in the necessary alignment signal as in FIG. 28A, which lowers the precision of the wave analysis for the alignment, causing a decrease of the alignment precision.